All mature blood cells arise from a rare and specialized population, the hematopoietic stem cells (HSCs), which exist mostly in a quiescent state. Cell division of HSCs results in both their proliferation and progressive differentiation into increasingly lineage-restricted mature blood cells, as well as maintenance of a small pool of HSCs that do not differentiate, but rather carry out hematopoiesis throughout the life of an organism. Due to the significance of HSC function, the elucidation of the signals that govern the balance between HSC self-renewal and differentiation is a paramount task. Interestingly, several studies have suggested an intimate balance between physiological hematopoiesis and induction of hematopoietic malignancy (leukemia) controlled by aberrant signaling that is able to transform HSC and progenitor cells. In agreement with this notion, we have recently identified the E3 ubiquitin ligase Fbw7 as an important tumor suppressor in acute lymphocytic leukemia (ALL). Fbw7 inactivating mutations are found in a large fraction of ALL patients and induce transformation due to the aberrant stability of several important oncogenes, including Notch1 and c-Myc. We have addressed the role of Fbw7 in HSC function using a novel conditional knock-out mouse model. We have found that deletion of Fbw7 specifically and rapidly affected the HSC compartment in a cell-autonomous manner. Fbw7-/- HSCs showed defective maintenance of quiescence, leading to impaired self-renewal and a severe loss of competitive repopulating capacity. Furthermore, genome-wide transcriptome studies of Fbw7-/- HSC indicated that Fbw7 regulates a global transcriptional signature associated with the quiescent, self-renewing HSC phenotype. In this application we: a) Identify HSC-specific protein substrates targeted by Fbw7 and playing important roles in HSC differentiation and function, b) address the universal function of Fbw7 in stem cell self-renewal by studying its role in embryonic stem cell function and c) study the effects of ALL Fbw7 missense mutations in hematopoiesis and HSC self- renewal.